High levels of dietary fat, particularly polyunsaturated fat (PUF), lead to an increased incidence of skin and mammary tumors in animals. Furthermore, investigations indicate that dietary fat can modulate several parameters of the immune status in mice such as response to T-cell mitogens, the percentage of immunoglobulin positive cells, and serum immunoglobulin levels. Because of this apparent association between dietary fat and regulation of immune status and the incidence of mammary tumors experiments are proposed to determine the influence of dietary fat concentration and saturation on: 1) immune responsiveness, particularly those associated with tumor cytolysis; 2) susceptibility of mammary tumor cells to cytotoxicity; and 3) relationships between immune response and mammary tumorigenesis. Thus, we hope to determine some of the mechanisms by which lipids might regulate immune function and tumorigenesis in mice. This in turn may lead to insights of how dietary fats may be manipulated to affect tumor regression.